Figure 8-10. UML symbols for a boundary class.

As boundary classes control the interaction between users and the system, actors can communicate only with boundary classes. Boundary classes serve as a shield to separate the internal part of the system from the external events that may affect the system and vice versa. Usually, boundary classes are used to model graphic user interfaces.

There is at the least one boundary class per each actor/use case pair. Figure 8-11 presents an example of using a boundary class, referred to as SimulationForm, to control the dialog between the actor, referred to as user and the use case Start Simulation. The behavior of the class SimulationForm should provide all the operations needed to start a simulation, such as entering

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